Mice for conditional hnRNPLL expression or deletion have been generated. HnRNPLL cDNA was knocked into the ROSA26 locus downstream of a lox P flanked STOP cassette. These mice were crossed to mice with Cre-recombinase expression from the CD19 promoter, leading to hnRNPLL expression early in B cell development with the expected effect on CD45 alternative splicing. However, we were unable to observe a significant effect on B cell development. We are examining these mice in greater detail in collaboration with Dr. Anjana Rao of the La Jolla Institute. We have further generated mice for Cre-recombinase mediated hnRNPLL deletion. Similar to the CD19 knockin mice, deletion of hnRNPLL in B cells of CD19-Cre mice did not impact B cell development. In contrast, our preliminary data suggest that deletion of hnRNPLL in T cells of Lck-Cre mice perturbs early thymic development. We are presently examining the mechanistic basis of altered T cell selection in response to hnRNPLL loss.